The present invention relates to a mixer for mixing an input signal with a tuned local oscillator frequency. In particular, the present invention relates to an RF mixer having a variable gain.
Communication receivers are used in a variety of applications, such as in satellite tuners. One example of a satellite tuner includes a mixer for mixing an amplified RF input signal with a tuned local oscillator frequency. The RF input signal is typically a differential voltage. Various types of RF mixers have been used.
One traditional type of RF mixer is known as a xe2x80x9cGilbert Mixerxe2x80x9d. One representation of a Gilbert mixer uses an NMOS differential transistor pair as a transconductor to convert the differential voltage RF input signal to a differential current, which is then mixed with a differential local oscillator signal through a pair of cross-coupled differential transistor pairs. The resulting currents at the outputs of the cross-coupled differential transistor pairs can be coupled to a positive voltage supply rail through load resistors to generate a differential voltage output signal.
As supply rail voltage levels continue to reduce with advances in semiconductor technology, the voltage drop requirements across the mixing cell restricts the available output voltage swing when the output voltage swing is developed across load resistors to VDD. In technologies having low supply voltages, the head room constraints can be improved through the use of modified mixing cells using current folding techniques. The currents developed at the outputs of the cross-coupled differential transistor pairs are mirrored to a pair of PMOS output transistors and then coupled to a negative voltage supply rail through the load resistors to produce a differential voltage output. In this type of mixer, the voltage drop across the transconductors no longer constrains the head room available for the output voltage swing.
Folding the current through a current mirror also allows the common mode DC output voltage and the circuit gain to be set to desired levels. The common mode DC output voltage is defined by the values of the load resistors, the level of current supplied to the transconductors in the mixing cell and the ratio of the areas of the transistors that form the current mirror. For each current output of the transconductors, the current mirror has an input reference transistor and an output transistor. Thus by designing the input reference transistors and the output transistors with certain area ratios, a desired fixed circuit gain can be achieved. For example, if the areas of the output transistors are a third less than the areas of the reference transistors, then the AC signal current and the DC current in each load resistor will attenuated by a third. In addition, since the DC current in each load resistor is attenuated by a third, the common mode DC output voltage is also reduced by a third as compared to the common mode output voltage that would be present if the reference transistors and output transistors in the current mirror had equal areas.
In certain applications, it is desirable to provide the overall tuning system with a variable gain. Since the gain through an RF mixer stage is traditionally fixed, additional circuit stages are required for providing the variable gain. Improved mixing circuits are therefore desired that are capable of contributing to an overall system variable gain requirement while maintaining a substantially DC common mode voltage at the output of the mixer.
One embodiment of the present invention is directed to an mixer which includes a differential signal input, a differential local oscillator input, a gain control input and a differential signal output. The mixer has a stepped signal gain from the differential signal input to the differential signal output which is variable from a first gain to a second, different gain as a function of the gain control input and has a substantially constant DC common mode output bias voltage at the differential signal output.
Another embodiment of the present invention is directed to an mixer which includes a differential voltage signal input, a differential voltage local oscillator input, a gain control input and a differential voltage signal output. First and second load resistors are coupled to respective terminals of the differential voltage signal output. An input stage mixes the differential voltage signal input with the differential voltage local oscillator input to produce a mixed differential current. A current mirror mirrors the mixed differential current into the first and second load resistors. A gain switching circuit switches current gain in the current mirror as a function of the gain control input while maintaining a common-mode DC current in the first and second load resistors substantially constant.
Another embodiment of the present invention is directed to a method of mixing a differential input signal with a differential local oscillator signal with gain. The method includes mixing a differential input signal with a differential local oscillator signal to produce a mixed differential current; mirroring the mixed differential current into first and second load resistors through a current mirror; switching signal gain through the current mirror while maintaining a common-mode DC current in the load resistors substantially constant.